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Abstract
Poly(ethylene terephthalate) (PET) materials with different molecular weights were isothermally crystallized from melt by systematically varying the temperature and duration of the treatment performed in the differential scanning calorimeter (DSC). Multiple endotherm peaks were observed on the subsequent heating thermograms that were separated from each other on the basis of their melting temperature versus crystallization temperature and melting temperature versus crystallinity function. By this new approach five sub-peak sets were identified and then comprehensively characterised. Wide-Angle X-Ray Diffraction (WAXD) analyses revealed that the identified sub-peak sets do not differ in crystalline forms. By analysing the crystallinity and the melting temperature of the sub-peak sets as a function of crystallization time, crystallization temperature and intrinsic viscosity, it was concluded that below the crystallization temperature of 460 K the sub-peak sets that were formed during primary or secondary crystallization transform partially or completely to a third sub-peak set during the heating run of the measurement, while above this temperature, the sub-peak set formed during primary crystallization gradually transforms to a more stable structure, with higher melting temperature. These formations and transformations are described with mathematically defined parameters as well.
Highlights
The usage of poly(ethylene terephthalate) (PET) still increases nowadays; in original form and as secondary raw material alike [1, 2]
The aim of this research is to investigate the morphology of isothermally crystallized PET and to qualitatively analyse the multiple melting peaks as a function of duration and temperature of isothermal crystallization, and in relation to the intrinsic viscosity (IV) representing the average molecular weight
The molecular weight (MW) of the material was modified in two ways: (a) MW was increased by solid-state polycondensation (SSP)
Summary
The usage of poly(ethylene terephthalate) (PET) still increases nowadays; in original form and as secondary raw material alike [1, 2]. Journal of Polymer Research (2020) 27: 372 of the final products the mechanical properties are crucial, which are greatly affected by the morphology and crystalline structure formed during processing [8, 9]. The crystallization of PET can be either temperature or strain induced process [13]. Temperature induced crystallization takes place when the temperature is kept between the glass transition temperature (Tg) and the melting temperature (Tm). In this case, the crystallization process is determined by the change in the Helmholtz free energy.
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